The results of a systematic experimental investigation on the methods of enhancing the regression rate in hydroxyl-terminated polybutadiene (HTPB) fuel used in an HTPB/gaseous oxygen hybrid motor are presented The effects of the addition of ammonium perchlorate (AP) or aluminum in the fuel, the variation of oxidizer ‐fuel ratio, and the variation of characteristic dimensions of fuel grain are presented For the extents of the parametric variations considered, while the addition of AP and/or Al and the reduction of grain port diameter enhance the regression rate, the effect due to the latter is the most signie cant one Furthermore, the regression rate increases along theaxis, and it becomes essentially constant in the port region corresponding to a fuel-rich composition The possible physical processes for all of these behaviors are discussed The experimentally obtained exponents of the variables for regression rate (oxidizer mass e ux and port diameter ) are found to be signie cantly different from those of the conventional theory The similarity between the fuel regression rate equation used in solid-fuel ramjet and that obtained in hybrid motor is discussed

Fuel Regression Rate in Hydroxyl-Terminated-Polybutadiene/ Gaseous-Oxygen Hybrid Rocket Motors

Hybrid rocket fuel average regression rate is one of the most important values to accurately determine in the hybrid rocket design process and for rocket performance prediction. Yet there is no comprehensive theory that can be used to reliably predict this quantity. Additionally, regression rate data is difficult to measure. Measured data often contains a high degree of scatter, suffers from scale effects and is generally a closely held secret by those performing the experiments and therefore is unavailable for many propellant combinations. This paper presents a regression rate model that has been developed based on the results of several previous studies. The model is applicable to vaporizing fuels in a cylindrical grain configuration that do not form significant char or melt layers. It accounts for the presence of a pre-combustion chamber upstream of the fuel grain and also for variable gas properties (to a limited degree). The model is compared with existing published regression rate data. The results of the comparison are reasonable given the level of approximation in the model but additional work is required before models of this type will supplant regression rate measurements for rocket design purposes.

Hybrid Rocket Fuel Regression Rate Data and Modeling

A series of firing tests was carried out to investigate the influence of the oxidizer injection on the solid fuel regression rate behavior in a hybrid rocket engine. For this purpose, a conical subsonic nozzle as the injector of the gaseous oxidizer was selected to generate nonuniform conditions at the entrance of the fuel port. Gaseous oxygen and polyethylene fuel cylindrical grains were used. When the oxygen was fed by this kind of injector, the fuel regression in the region of the oxygen impingement on the grain’s surface was increased several times, which led to irregular fuel consumption with the characteristic afterburn port shape typical of solid fuel ramjets having a rearward-facing step at the air inlet. The local instantaneous regression rates, measured by means of the ultrasound pulse-echo technique, showed regression rate-time profiles strongly dependent on the geometric configuration and helped to explain the complex regression phenomenon deriving from the impingement zone displacement during the time. Empirical correlations for the prediction of the average regression rate were developed taking into account the influence of mass flux, pressure, and port diameter. Finally, a nondimensional semi-empirical correlation involving the effect of the blowing number yielded improved accuracy.

Role of Injection in Hybrid Rockets Regression Rate Behavior

This paper was aimed at analyzing the static engine firings results obtained by means of a hybrid rocket where gaseous oxygen was supplied into axial-symmetric polyethylene cylindrical grains through two different injector configurations: an axial conical subsonic nozzle and a radial injector. The axial injector is considered interesting because of its easy design and the remarkable feature that it produces no significant pressure oscillations for the stabilizing effect due to the hot gas recirculation zone established within the combustion port. To take advantage of its qualities, the assessment of the regression rate variations under the flow field generated by this configuration is required. For the investigated set of operating conditions, the instantaneous regression rates exhibit a time-and-space dependence caused by the impinging jet zone dynamics, while the average regression rates are higher and less mass flux dependent than those achieved with the radial injection motor and expected from the turbulent flow through pipes. A comparison to the data from the radial injector was further drawn in terms of combustion efficiency, fuel consumption profiles, and combustion stability. The radial injector, at the same mass flux and pressure, produces lower regression rates, high pressure oscillations and worse combustion efficiency at the same L*, but more uniform fuel consumption.

Influence of a Conical Axial Injector on Hybrid Rocket Performance

Hybrid rocket engines are promising propulsion systems which present appealing features such as safety, low cost, and environmental friendliness. On the other hand, certain issues hamper the development hoped for. The present paper discusses approaches addressing improvements to one of the most important among these issues: low fuel regression rate. To highlight the consequence of such an issue and to better understand the concepts proposed, fundamentals are summarized. Two approaches are presented (multiport grain and high mixture ratio) which aim at reducing negative effects without enhancing regression rate. Furthermore, fuel material changes and nonconventional geometries of grain and/or injector are presented as methods to increase fuel regression rate. Although most of these approaches are still at the laboratory or concept scale, many of them are promising.

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Approaches to Low Fuel Regression Rate in Hybrid Rocket Engines

Solid Fuel Ramjet Combustor Design

Fuel regression rate enhancement studies in HTPB/GOX hybrid rocket motors

A ballistic trajectory in air of a powered projectile where the thrust always balances the drag is termed as a pseudovacuum trajectory. For a solid-fuel-ramjet (SFRJ)-powered gun-launched projectile this trajectory can be achieved by the control of engine mass-e ow rate, either by a bypass control of inlet air or by a regression rate controloffuel. Based on one-dimensional considerations,theproceduresfora preliminary designofthepropulsion system for an SFRJ-assisted gun-launched projectile and the methods to calculate the control requirements are presented. Using these, typical cone gurations of SFRJ suitable for 155-mm gun-launched projectiles are analyzed for different launch angles. The results indicate that the control requirements by either method are minimal and thus, demonstrate the self-throttling characteristics of SFRJ.

Design and Control of Solid-Fuel Ramjet for Pseudovacuum Trajectories

The theoretical and experimenIal studies on hybrid rocket motor combustion research are briefly reviewed and the need for a clear understanding of hybrid rocket fuel regression rate mechanism is brought out. A test facility established at the Indian Institute of Technology, Madras, for hybrid rocket motor research study is described.The results of an experimental study on hydroxyl terminated polybutadiene and gaseous oxygen hybrid rocket motor are presented. Fuel grains with ammonium perchlorate "additive" have shownenhanced oxidizermass flux dependence. Smallergrains have higher regression rates than those of the larger ones.

Philmon George

Papers

Fuel Regression Rate in Hydroxyl-Terminated-Polybutadiene/ Gaseous-Oxygen Hybrid Rocket Motors

Solid Fuel Ramjet Combustor Design

Fuel regression rate enhancement studies in HTPB/GOX hybrid rocket motors

Design and Control of Solid-Fuel Ramjet for Pseudovacuum Trajectories

Regression Rate Study in HTPB/GOX Hybrid Rocket Motors.